1. Field of the Invention
The field of the invention is that of seismic data acquisition. More specifically, the invention pertains to equipment for seafloor analysis.
The invention relates in particular to the oil prospecting industry using seismic methods but can be applied in other fields implementing seismic data acquisition networks in marine environments.
2. Background of the Invention
In the field of the invention, operations for on-site geological data acquisition conventionally use sensor arrays (known as “hydrophones” in the case of data acquisition in marine environments). Seismic data acquisition in a marine environment is conventionally done by means of a series of seismic streamers or linear acoustic antennas towed by a vessel and carrying hydrophones in particular.
It is now the common practice to conduct campaigns of marine seismic prospecting known as 3D prospecting where the network of towed streamers has a determined length and width and is towed at a controlled depth. Thus, the set of streamers is typically towed at a depth ranging from 5 m to 15 m. Streamers are formed by an assembly of sections generally about 150 m long, and each antenna may be several kilometers (6 km to 7 km, or even 10 km) long. Conventionally, the number of streamers deployed can go up to twelve (and it is probable that the number of streamers used will continue to increase in the future). Each streamer is equipped with seismic sensors and analog/digital conversion electronic circuitry associated with the sensors.
The vessel that tows the streamers also tow one or more seismic sources constituted by a network of air-guns or water-guns or again, acoustic vibrators. The pressure wave generated by the seismic source crosses the water column and insonifies the upper layers of the sea floor. One part of the signal is refracted by interfaces and the inhomogeneity of the ocean crust. The resulting acoustic signals are then detected by the seismic sensors distributed throughout the length of the streamers. These acoustic signals are conditioned, digitized and retransmitted by the telemetry of the streamers to an operator's station situated on the seismic vessel where the raw data is then processed.
To have an accurate image of the cartography of the seafloor of the zone explored, it is important to precisely locate the seismic sensors distributed along the streamers as well as the seismic source. Different techniques have been proposed in the prior art for the absolute localizing of the positions of the seismic sensors distributed along the streamers.
The localization of the marine streamers and of the seismic source initially used to be based on the use of GPS receivers and magnetic compasses. The GPS receivers were situated at a few particular points of the network, namely on the towing vessel, the support buoys of the seismic source and the head and tail buoys each connected to the streamers. The magnetic compasses distributed in greater numbers along the streamers were used to determine the deformations of the streamers between particular points.
More recently, better performing techniques for localization streamers have been proposed. These techniques still use GPS localization to get the absolute geographical referential system but with it they associate the use of underwater acoustics to determine the distances between the acoustic modules mounted along the streamers.
These acoustic antennas dedicated to the functions of localization streamers are mounted in line or clamped to the streamers. They may be transmitters and/or receivers used to determine the distances between neighboring modules situated on the adjacent streamers.
Hence, in order to obtain the precise localization of all the streamers, reference points are available. These are given firstly by the GPS receivers and secondly by a meshing of inter-module distances. These acoustic measurements are generally made regularly in a predefined acoustic sending and receiving sequence characterizing a predefined acoustic cycle. This acoustic cycle is defined at the start of sending and can be updated during changes in seismic prospecting lines. However, an acoustic cycle is not modified in the course of a line. Furthermore, the definition of the acoustic cycle is based on the nominal theoretical geometry (i.e. in the straight-line acquisition phase) of the device.
The acoustic cycle is optimized in order to reach a compromise between:                its duration: this duration must be as small as possible to increase the temporal precision of the measurements and consequently obtain a greater sampling frequency. It can be noted furthermore that the duration of the cycle is commonly sized to be smaller than the duration of a seismic acquisition, the latter duration depending on the topology of the marine subsurface which the seismic study must analyze as well as the productivity constraints of the seismic study;        the quality of the measurements: since the acoustic signals share the same propagation medium, the definition of the acoustic cycle must take account of the geometry of the acoustic network in order to avert risks of signal collision.        
As in radiofrequency transmission, there are various methods for sharing the transmission band:                frequency sharing;        sharing by means of transmission codes;        sharing in space (an acoustic wave continues to propagate after it has reached the target receiver);        sharing in time.        
That said, it can be noted in practice that a limitation of the cycle time always tends to cause deterioration in the quality of the acoustic measuring.
Along with the problem of the relative localization of the streamers with respect to one another, there is also the problem of controlling the streamers in depth.
Indeed, the depth of the streamers has a direct impact on the characteristics of the seismic signals received by the sensors. The depth is conventionally controlled by an appropriate adjustment of the floatability of the elements forming the streamer. This is done through the use of navigation control devices (commonly called aircraft or “birds”) as described by the patent document number FR-2 870 509. These devices are either attached to the streamers or inserted between two sections of the streamers.
Recently, seismic data acquisition techniques have developed into what are called 4D techniques: according to these techniques, the positioning of the streamer network is controlled in taking all three spatial dimensions (length, width, depth) and one temporal dimension into consideration. The time dimension is aimed at controlling the trajectory of the streamers in a zone considered, so as to reproduce trajectories already made on the same zone in a prior seismic data acquisition phase. These 4D techniques have propelled the development of new streamer control systems aimed at achieving both control of depth and control of the lateral position of the streamer. Lateral control necessitates a link with the acoustic localization system mentioned here above to provide the control device with information elements on relative distances between the antennas. These information elements are necessary for the control of the network. Here again, these new systems require increased precision in the localization measurements, including precision in acoustic localization techniques.
It must furthermore be noted that acoustic localization measurements are generally used only during seismic data acquisition phases.
Now since the configuration of an acoustic measuring cycle is static, it is generally not suited to phases in which the antennas are turning or are being deployed or folded. Indeed, since the time slots for sending and receiving acoustic signals are generally optimized for the nominal use case, changes in the geometry of the device will lead to time lags for acoustic receiving which then will no longer correspond to the time slots defined for the acoustic receiver elements.
When the phases no longer correspond to the nominal use case, the cables can then only be localized through the GPS localization of a tail buoy and by the use of compasses distributed along the cable. However, these elements by themselves cannot give a precise localization of the cable. However, these phases are generally difficult to implement and the possibility of adding a precise acoustic localization should appreciably improve the quality of localization of the cables and therefore the security of the device in water.